1. Field of the Invention
The present invention is directed to a joint gap measurement and restoration method and, more particularly, to a method which matches the surfaces of a joint to prevent leaks from a high pressure vessel such as a steam turbine.
2. Description of Related Art
During the normal operating period of a steam turbine, between maintenance outages, which occur approximately every five years, erosion may occur in and around the joint surfaces between the turbine high pressure cylinder cover and the cylinder base. When gaps between the joint surfaces occur, high pressure steam is lost from the turbine reducing turbine efficiency. The leaking steam also creates an avoidable but unwanted safety problem for people working around the turbine The steam is made of expensive high purity water causing additional economic losses over and above the efficiency losses previously mention above. Typical approaches to minimizing the creation of joint gaps and repairing the gaps include providing a sealant on the joint surfaces prior to mating and repairing the joint surfaces using a blue plating method. In the blue plating method, the surface of the base and the cover are separately processed to produce independently referenced flat surfaces. During this process a feeler gauge is used to detect the gaps in the joint prior to removal of the cover from the base. Once the cover is removed, each of the joint surfaces is coated with a prussian blue contact detection medium A relatively heavy surface plate approximately 11/2 feet by 4 feet on one of the joint surfaces When the plate is inverted the amount of prussian blue medium from the plate indicates the location of high spots and the relative flatness of the joint surface covered by the plate with respect to the plate. The joint surface is then repaired by filling in low spots with welding and taking off high spots. The high spots are traditionally removed with power scrapers, files, sanders and finishing stones. The process of applying the plate to the same location, examining the blue contact area and repairing the surface continues until approximately 100% of the plate is blue. The plate is then moved to another location and the process of producing a flat surface is continued. Once one surface of the joint is flat relative to the movable plate, the other joint surface is examined and repaired in the same manner. This prior art process creates a relatively flat surface with respect to the metal surface plate. Even with a relatively flat cover joint surface and a relatively flat base joint surface, the cover and base joint surfaces do not necessarily match and gaps may continue to exist Because the gap may not be at a minimum on the seal surfaces of the joint, erosion can cause bad gaps to develop more easily. A turbine is typically 30 feet long and 12 feet wide with a linearly measured joint surface of approximately 84 feet. The blue plate process is thus very time consuming because the entire surface of both the base and the cover of the turbine must be examined and repaired.